This invention generally relates to a telephone loudspeaker systems and more specifically is directed to a pseudo-full duplex telephone loudspeaker system especially adapted for use in a conventional television receiver.
Standard telephone communication is of the full duplex type providing for the simultaneous communication in both directions between the called and the calling parties. Full duplex communication, however, is not available in a loudspeaker telephone system because of its component configuration and resulting operating characteristics.
For example, audio signals emitted by the system's loudspeaker impinge on the microphone, either by direct or indirect accoustic coupling. This results in an accoustic echo signal being produced at the microphone resulting in the generation of self-oscillations in system circuitry which produces howling noises causing a degradation of the communications link.
In addition to this accoustic coupling between the loudspeaker output and the resulting unwanted input to the microphone, electrical coupling between the transmission and reception channels degrades loudspeaker telephone communications. Typically, a loudspeaker telephone system includes a signal transmission channel incorporating a high gain amplifier connecting a microphone to a two-wire telephone subscriber line. A reception channel also incorporating a high gain amplifier connects the subscriber line to the loudspeaker. A differential coupler, such as an isolation transformer, is used to connect the transmission and reception channels to the subscriber line, and is intended to prevent signal leakage between the transmission and reception channels. However, because local telephone line impedance varies from site to site and even from time to time as various load, such as additional telephone extensions, are imposed on the telephone network, the impedance of the differential coupler is seldom a perfect match with that of the telephone system. Consequently, the transmission signals break through into the reception channel resulting in an electrical echo signal being present in the reception channel.
Improved isolation between the transmission and reception channels is generally provided to reduce the electrical echo signal. One of the best known approaches to minimizing the effects of acoustic echoes in a loudspeaker telephone system involves introducing a certain level of attenuation into the transmission channel or the reception channel, depending on whether the distant or local end subscriber is speaking. This method uses so-called "voice-operated gain adjusting devices" to operate the equipment in alternate modes, i.e., in transmission or in reception.
One approach to an automatic switching device for a communications system is disclosed in U.S. Pat. No. 2,766,378 to Sundin et al wherein is described an automatic switching device actuated in response to a DC voltage exceeding a certain threshold potential and which for its duration causes an amplifier for a second signal to be non-conducting. Thus, a changeover takes place from the second signal to the first signal with the first signal acquiring predominance over the second signal by an increase in the gain of an amplifier for the first signal together with a simultaneous suppression of the second signal initiated by an audio input provided to the first signal's amplifier.
Another audio signal-actuated switching scheme is disclosed in U.S. Pat. No. 4,087,636 to Akiyama et al involving a transmit-receive circuit changing switch system which is operated by a signal produced by a switching signal generator from a low frequency signal produced by the operator's voice. The system is intended to eliminate the need for a press-to-talk manual switch, the function of which is accomplished by means of antenna and microphone-coupled transducer change-over switches controlled by an electroacoustic transducer change-over switch. The change-over signal generator circuit is responsive to a received audio signal and configures the system in a receiving mode of operation. When the received audio signal is no longer detected, the system reverts to a signal transmission mode of operation. The audio frequency signal provided at the output of the receiver is prevented from entering the switching signal generator by means of the transducer change-over switch which isolates the input of the audio frequency amplifier from the output of the receiver.
The above-discussed patents are examples of so-called "voice-operated gain adjusting devices" for operating communications equipment in alternating modes of receive and transmit. As such, these systems suffer from limitations generally inherent in such communication links. For example, if the operating mode switchover function operates at high speed, the conversation becomes unpleasant with speech being cut up into separate syllables and brief noises resulting in unnecessary switching operations. If the operating mode switching is too slow, the initial portion of a conversation will likely be lost, such as the first one or more syllables of a reply, with the other party not being able to break in and the reception channel of the speaking party being rendered totally inactive.
U.S. Pat. No. 4,225,754 to Bernard et al discloses a loudspeaker telephone system intended to eliminate spurious noises caused by circuit oscillation due to transmitreceive signal feedback. The transmission and reception channels are decoupled by means of first and second adaptive transversal signal filters incorporated in the reception and transmission channels. These adaptive transversal filters use the outputs of their respective channels to synthesize an acoustic echo signal component present in the output from that channel with its combining circuit subtracting the real and synthetic echo signal components from each other to reduce the acoustic echo signal component in the respective channel to a residual level. In an alternate embodiment, both transversal filters may be embodied in a single filter circuit operating on a time-sharing basis. U.S. Pat. No. 4,251,689 to Hastings-James et al involves a loudspeaker telephone apparatus wherein the loudspeakers are used as bi-directional receive and transmit transducers. Under normal conditions, the gain of the receive amplifier is at a maximum value and that of the transmit amplifier at a minimum value. Under quiescent conditions speech entering the microphone circuit switches the receive amplifier to its "off" condition and the transmit amplifier to its "on" condition. The set is thus voice-switched from the receive condition to the transmit condition. When the voice signal stops, the receive amplifier will return to its maximum gain or "on" and the transmit amplifier will return to minimum gain or "off". This is accomplished by means of a bridge circuit in the separate branches of which are connected the coils of the loudspeakers, one of which is coupled in a series-aiding configuration and the other is coupled in a series-bucking configuration. The telephone line is connected to the balanced bridge by means of the receive and transmit amplifiers. The amplifier to be turned on is deterined by the balanced status of the bridge circuit with one amplifier turned "on" when the bridge is balanced and the other turned "on" when the bridge is unbalanced. The referenced loudspeaker telephone systems are designed to address specific problems in such systems. The first apparatus is designed specifically to minimize speaker system oscillations caused by acoustic signal feedback while the second system is intended to provide a multi-point telephone conferencing system wherein no isolation between the transmit and receive signals is required since the loudspeaker is used for processing both signals.
The present invention is more general in its application than the aforementioned approaches and provides a pseudo-full duplex telephone loudspeaker communication system compatible with and easily installed in a conventional television receiver. Improved electrical and acoustical isolation between the transmit and receive signals is accomplished by means of a microprocessor which samples these signals at a high rate and provides signal amplitude corrections therefor.